Protective coating for magnesium



Patented Feb. 23, 1926.

UNITED STATES PATIENT OFFICE.

LEWIS J. KEELER, OF NIAGARA FALLS, NEW YORK, ASSIGNOR T0 AMERICAN MAG-NESIUM CORPORATION, OF NIAGARA FALLS, NEW YORK, A CORPORATION OF NEWYORK.

No Drawing.

To all whom it may concern:

Be it known that I, Lewis J. Kinsman, a citizen of the United States,residing at Niagara Falls, in the county of Niagara and State of NewYork, have invented certain new and useful Improvements in ProtectiveCoatings for Magnesium, of which the following is a specification.

My invention relates to a protective coating for magnesium metal andhigh magnesium alloys. The coating which I produce nesium.

by my process is practically transparent and microscopic in depth andpresents a surface having the characteristic appearance of mag- Thiscoating prevents the metal from tarnishing even under severe atmospheric conditions.

The coating is produced by immersing the metal in a solution ofhydrofluoric acid. This is preferably done by making the metal to becoated the anode in an electrolytic cell, in which [the electrolyte is ahydrofluoric acid solution. Although various strengths of the acid maybe used, an approximately 4.8% acid solution has been found to be themost suitable. The metal to be coated is first cleaned by some suitablemeans such as by scratch brushing or by dipping in acids, preferably indilute nitric acid. The nitric acid has the special advantage of givingthe metal a very bright luster. If acid is used in cleaning, the metalmust subsequently be washed well with running water. It is then readyfor the coating operation. The article is then made the positive poleand serves as the anode of the electrolytic cell. The cathode of thiscell may be constructed of any material which is an electricalconductor. I have found magnesium metal or carbon'to be best adapted forthis purpose.

At the beginning of the coating operation, the'resistance of the cell iscomparatively low. As the coating operation proceeds, the resistanceincreases rapidly and for this reason it has been found advisable tochoose a potential .of 110 volts or higher. A.,variable resistance mustbe placed in series with the cell in order to control the anode currentdensity. Although I do not limit myself to any definite anode currentdensity, a density of one ampere per 100 square inches of anode surfacehas been found to be very satisfactory. Due to change in volt-IPROTECTIVE COATING FOR M'AGNESIUM.

Application filed January 22, 1923. Serial No. 614,804.

age across the cell, as explained above, it will be evident that thecurrent will change. correspondingly unless the variable resistance isadjusted. For instance, it has been found that if the resistance is notadjusted, the current will rapidly drop from one ampere per 100 squareinches of anode surface to practically zero in a period of two to fiveminutes after starting the current.

The coating as formed on the metal from present information is believedto be either magnesium fluoride or oXy-fluoride or possibly a mixture ofthe two. The coating may also be obtained by simply allowing the metalto remain in a solution of hydrofluoric acid. The action is very slow,taking several weeks to obtaina fairly good protective coating, but thecharacteristics of the coating seem to be the same as those obtained inthe electrolytic bath. Water soluble fluoride salts, such as potassiumfluoride, may also be used as an electrolyte in place of hydrofluoricacid. In fact, any composition may be used which under suitableconditions will liberate nascent fluorine in contact with the metal tobe coated. Although I prefer to use direct current in the electrolyticcell, alternating current can also be used.

Havin described the various phenomena which ta e place in my process, Iwill now outline the details of procedure in coating a rod of magnesiumor magnesium alloy. The rod is first cleaned by dipping in dilute nitricacid for approximately one half minute. The nitric acid solutionremaining on the rod is then quickly washed off in running water and therod at once supmerged in the hydrofluoric acid electrolyte of the cell.Thepositive terminal of a 110 volt direct current circuit is attached tothe its alloys comprising bringing the metal in contact with nascentfluorine.

2. The process of coating magnesium and its alloys comprising bringingthe metal in contact with an aqueous: solution containing hydrofluoricacid.

3. The process of forming a coating on an article of magnesium or itsalloys, comprising employing the article as anode in the electrolysis ofan aqueoussolution containing a compound of fluorine.

4. The process of formin a coating on 10 an article of magnesium or 1tsalloys, comthe article as anode in the electro] sis an ac ucous solutioncontaining hy rofluoric aci 5. A magnesium or magnesium alloy articlehaving on its surface an adherent coating comprising magnesium fluoride,which is resistant to atmospheric corrosion.

In testimony whereof I afiix my signature.

LEWIS J. KEELER.

prising employin

